The present invention relates to an optical pickup for recording/reproducing information on/from an optical disc, especially, provided with a hologram element.
A known optical pickup is provided with a hologram element in accordance with an astigmatism method, aiming for both of focus error detection and compactness as well as high reliability.
Another known optical pickup is provided with two laser sources with different wavelengths for two types of optical disc with different recording densities, such as, CD and DVD. This type of optical disc is generally provided with a laser source dedicated for DVD that is an optical disc of high recording density used at shorter wavelength than CD and another laser source dedicated for CD, the former laser source being located on the optical axis of an objective lens, the latter being located in the vicinity of the former.
A still another known optical pickup is used for recording/reproducing information on/from an optical disc with two information recording layers, such as dual-layered DVD.
Discussed below with reference to FIGS. 1 to 3 are problems that occur: when a known optical pickup having a hologram element in accordance with the astigmatism method is provided with two types of laser source; and when such a known optical pickup is used for reproduction of an optical disc having two information recording layers.
FIG. 1 shows schematic illustration of a positional relationship between a known optical pickup, and an objective lens and an optical disc. In detail, (a) and (b) of FIG. 1 are sectional views illustrating reproduction of a DVD and a CD, respectively. A sectional view in (c) of FIG. 1 illustrates reproduction of a dual-layered DVD, in detail, from the information recording layer closer to laser sources.
FIG. 2 shows schematic illustration of a hologram section in the known optical pickup. In detail, (a) and (b) of FIG. 2 are plan views illustrating a beam on a hologram section, returned from a DVD and a CD, respectively, in reproduction. A plan view in (c) of FIG. 2 illustrates a beam on a hologram section, returned from a dual-layered DVD, in detail, from the information recording layer closer to laser sources.
FIG. 3 shows schematic illustration of a light receiving section in the known optical pickup. In detail, (a) and (b) of FIG. 3 are plan views illustrating a beam on a light receiving section, returned from a DVD and a CD, respectively, in reproduction, and after diffracted when passing through a hologram section. A plan view in (c) FIG. 3 illustrates a beam on a light receiving section, returned from a dual-layered DVD, in detail, from the information recording layer closer to laser sources, and after diffracted when passing through a hologram section.
As shown in FIG. 1, a known optical pickup 100 is provided with a laser source 101 for DVD use, a laser source 102 for CD use, a hologram element 103 having a hologram section 120, a collimator lens 104, and a photo receptor 105 having a light receiving section 130.
The laser source 101 for DVD use is located on the optical axis of an objective lens 200. The laser source 102 for CD use is located in the vicinity of the laser source 101 for DVD use. The distance between the hot spots of the laser sources 101 and 102 is 100 μm or more depending on the structure of the two laser sources.
As shown in FIG. 2, the hologram section 120 has six hologram zones 121 to 126. Chain double dashed lines in (a), (b) and (c) of FIG. 2 indicate tracks of optical discs D1, D2 and D3, respectively.
As shown in FIG. 3, the light receiving section 130 has four light receiving zones 131 to 134.
Discussed below first is a problem related to the two types of laser source used in the known optical pickup 100.
In reproduction of a DVD, an optical disc D1 in (a) of FIG. 1, as shown in (a) of FIG. 2, a beam spot Lb111 of a return beam Lb101 from the optical disc D1 is located in the center region of the hologram section 120. This is because the laser source 101 for DVD use is located on the optical axis of the objective lens 200, as shown in (a) of FIG. 1.
The return beam Lb101 is diffracted when passing through each of the six hologram sections 121 to 126. The diffracted return beam Lb101 then reaches the photoreceptor 130, as shown in (a) of FIG. 1.
Beam areas 141 to 146 on the photoreceptor 130, shown in (a) of FIG. 3, are particular regions on which the return beam Lb101 reaches after diffracted when passing through the hologram sections 121 to 126 in (a) of FIG. 2.
The beam areas 141 to 146, shown in (a) of FIG. 3, correspond to the hologram sections 121 to 126, shown in (a) of FIG. 2, respectively.
In reproduction of a CD, an optical disc D2 in (b) of FIG. 1, as shown in (b) of FIG. 2, a beam spot Lb112 of a return beam Lb102 from the optical disc D2 is shifted from the center of the hologram section 120. This is because the laser source 102 for CD use is shifted from the optical axis of the objective lens 200, as shown in (b) of FIG. 2. The center OLb112 of the beam spot Lb112 is located in a zone different from the hologram zones 122 and 125.
The return beam Lb102 is diffracted when passing through each of the six hologram sections 121 to 126. The diffracted return beam Lb102 then reaches the photoreceptor 130, as shown in (b) of FIG. 1.
Beam areas 151 to 156 on the photoreceptor 130, shown in (b) of FIG. 3, are particular regions on which the return beam Lb 102 reaches after diffracted when passing through the hologram sections 121 to 126 in (b) of FIG. 2.
The beam areas 151 to 156, shown in (b) of FIG. 3, correspond to the hologram sections 121 to 126, shown in (b) of FIG. 2, respectively.
As shown in (a) and (b) of FIG. 3, the beam areas 155 and 152 in CD-reproduction are smaller than the beam areas 145 and 142 in DVD-reproduction, in the light receiving zones 131 and 132. The size of the beam areas corresponds to the amount of light of the beam spot Lb111 in (a) of FIG. 2 and of the beam spot Lb112 in (b) of FIG. 2, on the light receiving zones 131 and 132. Thus, in (b) of FIG. 3, the light receiving zones 131 and 132 receive a smaller amount of light due to the smaller beam areas 155 and 152, resulting in that the photo receptor 105 having these light receiving zones 131 and 132 exhibits lower focus-servo error detection sensitivity in CD-reproduction.
The locations of beam areas on the light receiving section 130 depend on the positional accuracy of optical components in optical pickup assembly.
Broken lines in (a) of FIG. 3 indicate shifted beam areas 145 and 142, in DVD-reproduction, due to inaccurate locations of optical components in optical pickup assembly. Likewise, broken lines in (b) of FIG. 3 indicate shifted beam areas 155 and 152, in CD-reproduction, due to inaccurate locations of optical components in optical pickup assembly.
As shown in (a) of FIG. 3, the shifted beam areas 145 and 142 indicated by the broken lines have a shorter length of superposition with a split line x130 of the light receiving section 130 than the beam areas 145 and 142 (indicated by solid lines) with no shifting. This results in lower focus-servo error detection sensitivity of the photo receptor 105 in DVD-reproduction, for the same reason as discussed above.
Moreover, As shown in (b) of FIG. 3, the shifted beam areas 155 and 152 indicated by the broken lines have a shorter length of superposition with the split line x130 of the light receiving section 130 than the beam areas 155 and 152 (indicated by solid lines) with no shifting, which is much shorter than the shifted beam areas 145 and 142 in (a) of FIG. 3. This results in further lower focus-servo error detection sensitivity of the photo receptor 105 in CD-reproduction, for the same reason as discussed above.
Such lower focus-servo error detection sensitivity could cause delay in tracking of a fluctuating optical disc by the objective lens 104 in recording/reproduction in FIG. 1, which causes a lot of jitters in recorded/reproduced signals or tracking errors.
Discussed next is a problem concerning reproduction of a dual-layered optical disc, such as, a dual-layered DVD, in the known optical pickup 100.
As shown in (c) of FIG. 1, an optical disc D3, such as a dual-layered DVD, has two information recording layers d3a and d3b. 
In reproduction of the information recording layer d3a of the optical disc D3, closer to the optical pickup 100, a beam spot Lb113 of a return beam Lb103 from the information recording layer d3a is located in the center region of the hologram section 120, as shown in (c) of FIG. 2. The center region of the hologram section 120 is a zone with astigmatism applied for focus error detection.
During the reproduction of the information recording layer d3a of the optical disc D3, a return beam Lb104 from the information recording layer d3b not undergoing reproduction also reaches the hologram section 120. The return beam Lb104 is an unwanted beam but forms a beam spot Lb114, as indicated by a small solid-line circle in (c) of FIG. 2.
The unwanted return beam Lb104 is diffracted when passing through each of the six hologram sections 121 to 126. The diffracted return beam Lb104 then reaches the photoreceptor 130, as shown in (c) of FIG. 1.
The unwanted return beam Lb104 reaches beam areas 161 to 166, shown in (c) of FIG. 3, after undergoing diffraction by each of the six hologram sections 121 to 126. The beam areas 161 to 166 are the regions on the photoreceptor 130 and the neighboring regions.
The beam areas 161 to 166, shown in (c) of FIG. 3, correspond to the hologram sections 121 to 126, shown in (c) of FIG. 2, respectively.
Tracking, while the return beam Lb103 and the unwanted return beam Lb104 are reaching the hologram regions 121 to 126, causes the beam spot Lb114 of the unwanted beam Lb104 being shifted left and right while crossing split lines A120 and B120. This is indicated by a small broken-line arcs that surround the small solid-line circle (that indicates the beam spot Lb114) in (c) of FIG. 2.
The shift of the beam spot Lb114 of the unwanted return beam Lb104 discussed above results in out-of-focus and the distribution of beam areas 161 to 166, as shown in (c) of FIG. 3.
It is understood from (c) of FIG. 3 that the sum of the beam areas 161 to 166, that is the sum of the amount of light on the beam areas 161 to 166, is different among the light receiving zones 131 to 134. This means that the amount of light of the unwanted return beam Lb104 is different among the light receiving zones 131 to 134, which further means that the influence of the unwanted return beam Lb104 on the light sensitivity of the light receiving zones 131 to 134 is different among the light receiving zones. This results in noises being added to a focus-error detection signal in the photo receptor 105, which causes out-of-focus recording/reproduction with a lot of jitters in recorded/reproduced signals.